It is known that oxygen debt within the tissues supplied by the retinal artery is associated with changes to the inner retina Acute changes include swelling of the inner nuclear, inner plexiform, and ganglion cell layers. Typically, in a matter of days pyknotic nuclei are observed, which is followed by loss of ganglion cells and a thinning of the inner retina. Many of the histological changes observed in the inner retina following hypoxia were elucidated using the chick embryo retina preparation. David used this model to show that when subjected to anoxia, morphological changes occurred that were similar to the toxic effects of glutamate or potassium administration David, P., et al., "Involvement of excitatory neurotansmitters in the damage produced in chick embryo retinas by anoxia and extracellular high potassium," Exp. Eye Res. 46: 65762, 1988). Immunocytochemical and electrophysiological studies have unequivocally proven that glutamate is the major excitatory neurotansmitter in the retina as well as most other regions in the central nervous system (Massey, S., "Cell types using glutamate as a neurotansmitter in the verebrate retina," Progress in Retinal Research, Volume 9, edited by N. N. Osborne and G. J. Chader, Oxford: Pergammon Press, 399-425, 1990; Miller, R. F. and M. M. Slaughter, "Excitatory amino acid receptors in the vertebrate retina," Retinal Transmitters and Modulators, Volume II, edited by W. W. Morgan. Boca Raton: CRC Press, Inc. 123-160, 1985). Under normal conditions neuronal release of glutamate is presynaptic and mediates an excitatory response on post-synaptic excitatory amino acid (EAA) receptors. Dissociation from these receptors as well as uptake into presynaptic neurons and/or glial cells is very rapid. Under certain conditions glutamate release can be excessive and uptake mechanisms compromised. In the central nervous system it has been demonstrated that brief periods of ischemia can rapidly increase glutamate levels. Beneviste demonstrated increases in glutamate in the adult rat brain within minutes of global cerebral ischemia (Beneviste, H. et al., "Elevation of the extracellular concentrations of glutamate and aspartate in rat hippocampus during transient cerebral ischemia monitored by intracerebral microdialysis," J. Neurochem. 43(5): 1369-1374, 1984.).
Excessive EAA stimulation, referred to as excitotoxicity, can result in neuronal injury (Olney, J. W., "Inciting excitotoxic cytocide among central neurons," Adv. Exp. Med Biol. 203:631-645, 1986). The process of excitotoxicity has been extensively studied in the retina Such toxicity to the inner retina has been observed following intravitreal injection of EAAs, following application of EAAs to the isolated animal retina, or from exogenously applied glutamate to retinal ganglion cells in culture. See generally, Sattayasai, et al., "Morphology of quisqualate-induced neurotoxicity in the chicken retina," Invest. Ophthalmol. Vis. Sci., 28:106-117 (1987); Tung, et al., "A quantitative analysis of the effects of excitatory neurotoxins on retinal ganglion cells in the chick," Visual Neurosci., 4:217-223 (1990); Sisk, et al., "Histological changes in the inner retina of albino rats following intravitreal injection of monosodium L-glutamate," Graefe's Arch. Clin. Exp. Ophthamol., 223:250-258 (1985); Siliprandi, et al., "N-methyl-D-aspartate-induced neurotoxicity in the adult rat retina," Visual Neurosci., 8:567-573 (1992); Reif-Lehrer, et al., "Effects of monosodium glutamate on chick embryo retina in culture," Invest. Ophthalmol. Vis. Sci., 14(2):114-124 (1975); Blanks, J. C., "Effects of monosodium glutamate on the isolated retina of the chick embryo as a function of age: A morphological study," Exp. Eye Res., 32:105-124 (1981); Olney, et al., "The role of specific ions in glutamate neurotoxicity," Neurosci. Lett., 65:65-71 (1986); Olney, et al., "The anti-excitotoxic effects of certain anesthetics, analgesics and sedative-hypnotics,", Neurosci. Lett. 68:29-34 (1986); Price, et al., "CNQX potently and selectively blocks kainate excitotoxicity in the chick embryo retina," Soc. Neurosci. Abst., 14:418 (1988); David, et al., "Involvement of excitatory neurotansmitters in the damage produced in chick embryo retinas by anoxia and extracellular high potassium," Exp. Eye Res., 46:657-662 (1988); Caprioli, et al., "Large retinal ganglion cells are more susceptible to excitotoxic and hypoxic injury than small cells," Invest. Ophthalmol. Vis. Sci., 34(Suppl):1429 (1993); Cummins, et al., "Electrophysiology of cultured retinal ganglion cells to investigate basic mechanics of damage," Glaucoma Update IV, 59-65 (1991); and Sucher, et al., "N-methyl-D-aspartate antagonists prevent kainate neurotoxicity in rat retinal ganglion cells in vitro," J. Neurosci., 11(4):966-971 (1991).
EAA receptors have been characterized as metabotropic or ionotropic. Activation of a metabotropic receptor affects cellular processes via G-proteins; whereas ionotropic receptors affect the tanslocation of mono- and divalent cations across the cell membrane. There are at least free ionotropic receptors that have been named for the agonist that preferentially stimulates the receptor. These receptors have been classified as: N-methyl-D-aspartate (NMDA); kainate; and 2-amino-3-(3-hydroxy-5- methylisoxazol- 4-yl) propanoic acid (AMPA) receptors. These EAA receptors are differentially distributed in specific cells in the retina (See, for example, Massey, S., "Cell types using glutamate as a neurotransmitter in the vertebrate retina," N. N. Osborne and G. J. Chader (Eds.) Progress in Retinal Research, Ch 9, Pergammon Press: Oxford, 399-425 (1990); and Miller, et al., "Excitatory amino acid receptors in the vertebrate retina," Retinal Transmitters and Modulators: Models for the Brain, (W. W. Morgan, Ed.) CRC Press, Inc., Boca Raton, II: 123-160 (1985).) The localization of such receptors accounts for the pathologies associated with ischemia of the retina or optic nerve head. For example, death of the retinal ganglion cell induced by kainate has to a large part been attributed to the NMDA receptor. (See, for example, Sucher, et al., "N-methyl-D-aspartate antagonists prevent kainate neurotoxicity in retinal ganglion cells in vitro," J. Neurosci., 11(4):966-971 (1991).).